The Chromosomal Basis of Inheritance AP Biology Chapter 15

Chromosome Theory of Inhertiance Locus: location of a gene on a chromosome Allele: different form of a gene Chromosome: – Segregation – Independent Assortment

The Chromosomal Basis of Mendel’s laws Figure 15.2 Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) Meiosis Fertilization Gametes All F 1 plants produce yellow-round seeds (YyRr) P Generation F 1 Generation Meiosis Two equally probable arrangements of chromosomes at metaphase I LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT Anaphase I Metaphase II Fertilization among the F 1 plants 9: 3 : YR yr yR Gametes Y R R Y y r r y R Y yr R y Y r R y Y r R Y r y rR Y y R Y r y R Y Y R R Y r y r y R y r Y r Y r Y r Y R y R y R y r Y F 2 Generation Starting with two true-breeding pea plants, we follow two genes through the F 1 and F 2 generations. The two genes specify seed color (allele Y for yellow and allele y for green) and seed shape (allele R for round and allele r for wrinkled). These two genes are on different chromosomes. (Peas have seven chromosome pairs, but only two pairs are illustrated here.) The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus. 1 Each gamete gets one long chromosome with either the R or r allele. 2 Fertilization recombines the R and r alleles at random. 3 Alleles at both loci segregate in anaphase I, yielding four types of daughter cells depending on the chromosome arrangement at metaphase I. Compare the arrangement of the R and r alleles in the cells on the left and right 1 Each gamete gets a long and a short chromosome in one of four allele combinations. 2 Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation. 3

Thomas Hunt Morgan & Fruit Flies Drosophila melanogaster – 2 week generations – 4 Chromosomes (3 autosomes & one pair of sex chromosomes) – Easy to breed Wild Type, what exists in nature has a + superscript – W + red eyes (wild type) – W white eyes (mutant) – Sex linked: on X chromosome in F 2 generation, only males showed the white eyes If not sex linked, ½ White eyes female & ½ white eyes male – If it weren’t sex linked, the F 2 white eyed flies would include females ocbio0115_.png

CONCLUSION Since all F 1 offspring had red eyes, the mutant white-eye trait (w) must be recessive to the wild-type red-eye trait (w + ). Since the recessive trait—white eyes—was expressed only in males in the F 2 generation, Morgan hypothesized that the eye-color gene is located on the X chromosome and that there is no corresponding locus on the Y chromosome, as diagrammed here. P Generation F 1 Generation F 2 Generation Ova (eggs) Ova (eggs) Sperm X X X X Y W W+W+ W+W+ W W+W+ W+W+ W+W+ W+W+ W+W+ W+W+ W+W+ W+W+ W W+W+ W W W

Sex Linked Genes Carried on the X chromosome – Heterozygous females are carriers they can pass allele to offspring w/out showing symptoms Females get disease if they inherit both recessive X alleles – Men are more likely to have diseases because they only have 1 X chromosome Duchenne Musculare dystrophy Color Blindness ALD Hemophilia

Recall: Crossing Over Causes recombination in linked genes Occurs during prophase I of meiosis between non- sister chromatids of homologous chromosomes – Recombinant frequency

Linked Genes Each chromosome contains hundreds or thousands of genes Genes on the same chromosomes are usually inherited together – linked gene – Not inherited together if crossing over takes place

Parental Types vs. Recombinants Parental Type - The offspring's phenotype resembles the parents Recombinant – Offspring’s Genotype is different from parental & Offspring’s genotype is new combination of parental genes 50% each = non linked – Caused by Independent assortment – genetic recombination Less than 50% recombinants = linked genes

Linkage Mapping The further apart 2 genes are on a chromosome the greater the chance of crossing over – greater recombinant frequency Map unit = recombinant frequency A frequency below 50% indicates that 2 genes are carried on the same chromosome aka linked

Linkage Mapping 1% recombination frequency = 1 map unit Recombination Frequencies A/B = 19% B/C = 4% A/D = 12% B/D = 23% What is the correct order of the genes? BACD

X Inactivation In mammals only one X chromosome is expressed in somatic cells Second X condenses to become a barr body Barr bodies are reactivated during gamete formation Tortoiseshell Cats – Female: orange fur where one X chromosome is expressed, black fur where other X is expressed

Sex Chromosomes X-Y system – mammals – XX = female – XY = male X-O system – some insects – grasshoppers, cockroaches – XX = female – X = male (sperm contained no sex chromosome Z-W system – birds, fish, some insects – ZW = female (determines sex) – ZZ = male Haplo-diploid system – bees and ants – no sex chromosomes – Diploid – females – Haploid – males (haploid, parthenogenic development) Figure 15.9b–d 22 + XX 22 + X 76 + ZZ 76 + ZW 16 (Haploid) 16 (Diploid) (b) The X–0 system (c) The Z–W system (d) The haplo-diploid system

Nondisjunction Homologous chromosomes do not separate in anaphase of meiosis Result is aneuploidy – Trisomy – 3 chromosomes n + 1 – Monosomy – 1 chromosome n – 1 – Polyploidy – 3 or more chromosomes

Chromosomal Alterations A B CD E FG H Deletion A B C E G H F A B CD E FG H Duplication A B C B D E C F G H A A MN OPQR B CD EFGH B CDEFGH Inversion Reciprocal translocation A BPQ R M NOCDEF G H A D CBEFH G

Chromosomes Alterations TypeExplanationExample 1. DeletionRemoval of a chromosome segment ABCDE  ABDE 2. DuplicationRepetition of a chromosomal segment ABCDE  ABBCDE 3. InversionReversal of a chromosome segment ABCDE  ABDCE 4. TranslocationMovement of a segment on one chromosome to another - nonhomologous ABCDE  FGCDE FGHIJ  ABHIJ

Full Chromosomal Disorders Syndromes – Down Syndrome – Trisomy 21 – Kleinfelters – XXY Sterile – Trisomy X – XXX – Turner syndrome – XO Sterile

Altered Chromosomal Disorders Cri du Chat – Deletion on chromosome 5 Leukemia – CML – Reciprocal translocation between chromosome 9 and 22

Genomic Imprinting Genes on autosomal chromosomes that are expressed depending on whether they come from the mother or father Insulin growth factor – only the paternal gene is activated

Organelle Inheritance Chloroplasts are inherited through the cytoplasm from the egg NOT the pollen Mitochondria are also passed in the cytoplasm of the egg – Not carried in sperm cells